Describe gene pyramiding and gene introgression with their importance in developing disease resistant varieties.

Gene Pyramiding: Stacking Resistance Genes

Gene pyramiding refers to the process of combining multiple, independently inherited resistance genes into a single plant variety. Each gene confers resistance to a specific pathogen or pest. By stacking these genes, a plant gains broader and more durable resistance. This is particularly important as pathogens evolve and overcome single-gene resistance.

Mechanism and Process

The process typically involves:

• Identifying Resistance Genes: Initially, genes conferring resistance to specific diseases are identified in different plant varieties or wild relatives.
• Marker-Assisted Selection (MAS): Molecular markers linked to these resistance genes are used to select plants carrying the desired combination of genes during breeding. This significantly speeds up the process compared to traditional phenotypic selection.
• Backcrossing: Backcrossing is often employed to recover desirable agronomic traits lost during the initial gene transfer.

Importance in Disease Resistance

Gene pyramiding offers several advantages:

• Broad Spectrum Resistance: Provides resistance against a wider range of pathogen races.
• Durability: Resistance is more durable as it's harder for pathogens to overcome multiple genes simultaneously.
• Reduced Reliance on Pesticides: Minimizes the need for chemical interventions.

Example

A classic example is the development of wheat varieties resistant to stem rust. Several resistance genes (e.g., Sr36, Sr51, Sr99) have been pyramided into commercial wheat varieties to provide long-lasting resistance to this devastating disease. The International Maize and Wheat Improvement Center (CIMMYT) has been instrumental in developing such varieties.

Gene Introgression: Introducing Genes from Wild Relatives

Gene introgression, also known as alien introgression, is the process of introducing genetic material from a wild relative or another species into a cultivated crop. Wild relatives often possess valuable traits, including disease resistance, that are absent in cultivated varieties.

Mechanism and Process

The process typically involves:

• Hybridization: Crossing the cultivated crop with its wild relative. This often results in sterile or poorly fertile offspring due to genetic incompatibility.
• Chromosome Doubling (Polyploidy): Treating the hybrid offspring with colchicine, a chemical that inhibits spindle formation during cell division, leading to chromosome doubling and restoring fertility.
• Selection and Backcrossing: Repeated backcrossing of the polyploid offspring to the cultivated crop, selecting for desirable traits from the wild relative while eliminating undesirable traits.

Importance in Disease Resistance

Gene introgression is crucial when resistance genes are unavailable in the cultivated species.

• Access to Novel Resistance: Provides access to resistance genes not present in the cultivated germplasm.
• Adaptation to Harsh Environments: Wild relatives often possess genes for tolerance to abiotic stresses like drought and salinity, which can indirectly enhance disease resistance.
• Genetic Diversity: Increases genetic diversity within crop populations, making them more resilient to evolving pathogens.

Example

The introduction of genes for resistance to white rust from *Brassica juncea* into *Brassica napus* (rapeseed/canola) is a significant example of gene introgression. This provided canola varieties with resistance to a disease that had previously caused significant yield losses.

Comparison: Gene Pyramiding vs. Gene Introgression

Feature	Gene Pyramiding	Gene Introgression
Source of Genes	Within the same species or closely related varieties	Wild relatives or distantly related species
Complexity	Relatively simpler, utilizes existing genes	More complex, involves overcoming genetic incompatibility
Time Required	Generally faster	Generally slower, requires multiple generations of backcrossing
Risk of Undesirable Traits	Lower risk, as genes are from similar genetic backgrounds	Higher risk, as alien genes can introduce undesirable traits

Challenges and Future Prospects

While both techniques offer significant advantages, they also face challenges. Gene pyramiding can be limited by the availability of suitable resistance genes and the complexity of combining them. Gene introgression can be hampered by genetic incompatibility and the risk of introducing undesirable traits. Advances in genomics and gene editing tools, such as CRISPR-Cas9, are revolutionizing these processes, allowing for more precise gene targeting and reducing the time required for breeding. The development of genome-edited varieties is also gaining regulatory acceptance in some countries, potentially accelerating the adoption of these techniques.

The Indian Council of Agricultural Research (ICAR) is actively involved in developing disease-resistant varieties using both gene pyramiding and gene introgression techniques. The National Bureau of Plant Genetic Resources (NBPGR) plays a vital role in conserving and characterizing germplasm for these breeding programs.

Gene pyramiding and gene introgression are vital tools in the arsenal against plant diseases, ensuring food security in a changing climate. While challenges remain, ongoing research and technological advancements, particularly in the field of gene editing, promise to further enhance the efficiency and precision of these breeding techniques. A concerted effort involving research institutions, breeders, and policymakers is crucial to harness the full potential of these approaches and safeguard our crops against emerging threats.